The present invention relates generally to a method and apparatus for implementation of digital modulation circuits, and specifically, to a method and apparatus for variably modulating a digital input signal in such circuits.
The dramatic increase in the capabilities and use of digital electronic products such as computers, radios, compact disk players, and cell phones has resulted in increased need for inexpensive digital electronic circuits that operate at higher and higher speeds. One such circuit is a sigma-delta modulator.
Sigma-delta modulators are use in a wide variety of electronic products. One such use is in a digital-to-analog converter in a compact disk player. The digital sigma-delta modulator is used to convert 2×16-bit words read from the compact disk at 44.1 kHz into 2×1-bit words at 16 times the original rate. The 1-bit words are converted into an analog signal using a 1-bit digital-to-analog converter, which is much easier to fabricate and make accurate than a 16-bit digital-to-analog converter.
Another use of sigma-delta modulators is in frequency synthesizers. Frequency synthesizers are digital electronic circuits that generate signals having different frequencies than the frequency of the input signal. In this application, sigma-delta modulators are used to provide a temporally-varying divide ratio sequence to control the division ratio of a frequency divider in the phase-locked loop. The division ratio determines the frequency ratio between the output signal and the input signal of the phase-locked loop.
Although sigma-delta modulators have proven useful in applications such as those exemplified above, they have some drawbacks. Attempts have been made to use higher-order sigma-delta modulators to attain better noise shaping characteristics. The size and power consumption of higher-order sigma-delta modulators, however, increase dramatically as the order and resolution increases. The increase in circuit size and power consumption is the result of the need for more and larger multipliers in higher-order sigma-delta modulators. Consequently, higher-order sigma-delta modulators are typically complex and expensive to manufacture.
Although attempts have been made to deal with the problems associated with the use of higher-order sigma-delta modulators, existing solutions are complex and expensive to implement due to the complexity of the circuitry required. Solutions to these problems have long been sought but prior developments have not taught or suggested any solutions and, thus, solutions to these problems have long eluded those skilled in the art.